Spindle longer than the barrel.

[rock_breaker]

A spent cartridge case would not be a good enough fit in the chamber because it springs back a bit after firing. Also, the primer pocket might not be concentric. The case wall is rather thin and might not hold the pressures from turning. Granted the chamber end is less critical when cutting at the muzzle but it would be better to use the valve stems to make inserts.
If you are turning the barrel between centres what are the copper jaw softeners for? the drive dog?

Yes they are for the drive dog and 4J marking up the barrel and aluminum for the muzzle brake. Have some brass that my work for protecting internal barrel holding. Right now the aluminum is mounted in the 4 jaw and protected by copper.
Not sure how to mount an outboard spider on the chamber end of the barrel when it is inside the spindle Bamban. I have made a center that may give me enough room to thread the muzzle just outside the 4 jaw. At this point I'm gathering do and don't information about this project. I am getting some good info on barrel alignment and protection.
Have a good day
Ray